Electrical motor circuits

ABSTRACT

A motor which is pulse-operated by chopper circuits connected to its field windings has a safety circuit which has two possible oscillatory loops. One loop is coupled to thyristors in the chopper circuit and another loop is coupled to the output of an armature current sensor. Bias circuits disable the loops when the thyristors are conductive and armature current is too high. A normally closed relay is kept closed only when the safety circuit oscillates. Cessation of oscillations opens the relay which thereupon disables the motor. The safety circuit is selfchecking; if it is itself inoperative the motor is still disabled.

O United States Patent 11 1 1111 3,764,873

Elliott Oct. 9, 1973 [5 ELECTRICAL MOTOR CIRCUITS 3,504,257 3/1970 Smith318/252 Inventor: Royston David Elliott Basingstoke, 3,424,959 1/1969Gruner 318/613 England Primary Examiner-Bernard A. Gilheany [73]Asslgnee: 'l 3 Assistant ExaminerW. E. Duncanson, Jr.

Basmgshll'e England Attorney-John W. Malley et al. [22] Filed: Feb. 29,1972 [21] Appl. No.: 230,288 [57] ABSTRACT A motor which ispulse-operated by chopper circuits [30] Foreign Application Pri rit D tconnected to its field windings has a safety circuit Apr. 4, 1971 GreatBritain 6 044 71 which has Pmslble dummy one loop is coupled tothyristors in the chopper circuit and an- 52 us. c1. 318/252, 318/366 iscoupled the Output ah armature 511 Int. Cl. II02 1/22 current sehsor-Bias circuits disable the when [58] Field of Search 3l8/l38 139 252,thyristors are hductive and armature current is 3 /297 30 3 3 7 434 4712 13 tOO high. A normally closed relay lS kept closed only when thesafety circuit oscillates. Cessation of oscillations opens the relaywhich thereupon disables the [56] References Cited motor. The safetycircuit is self-checking; if it is itself UNITED STATES PATENTSinoperative the motor is still disabled.

3,536,980 10/1970 McKenzie 318/6l3 6 Claims, 2 Drawing Figures CURRENTSENSING CIRCUIT TRIGGER ClREUlT PATENIEU 91175 3,764,873

SHEET 1 [IF 2 PATENTEU BET 75 SHEET 2 BF 2 Qw m 4 RN MN 4 53%. :35 53% 15% m? 25 1232s: 1232s: 25 \N mw 52:52 mm mm \W.\ v ww ELECTRICAL MOTORCIRCUITS BACKGROUND OF THE INVENTION This invention relates to safetycircuits for pulseoperated electric motors.

As is well known a pulse-operated electric motor is normally controlledby thyristors or other electronic or semi-conductive valves which arefed with operating pulses and thereby chop the supply of current to themotor into intermittent pulses. The specifications of US. Pat. Nos.3441827, 3562616 and 3601658 describe examples of such arrangements. Theelectrical circuit for controlling such a motor must normally include asafety circuit which prevents the driving of the motor if a particularfault occurs, and the aforementioned patents describe some examples.

Although many different safety circuits can be incorporated, thereremains a serious problem, in that a safety circuit itself may be orbecome faulty and fail to detect a fault in the circuit which itmonitors or fail to de-energise the motor.

SUMMARY OF THE INVENTION The present invention is accordingly concernedwith the provision of a self-checking safety circuit which includes atleast one oscillatory loop coupled to the circuit that it monitors andto a relay for disabling the motor.

Relatively high frequency signals can traverse the loop only if theelements and the safety circuit itself are in correct operativeconditions. These signals will not adversely affect the normal operationof the motor and its circuit. Should any fault, even in the safetycircuit itself, occur, the oscillations stop and the relay operates todisable the motor.

The main conditions that must be monitored by a safety circuit consistnormally of an excessive armature current and the conduction of theswitching elements in the chopper circuit. For this purpose the safetycircuit may have two oscillatory loops, one coupled to the choppercircuits and one coupled to respond to the output of an armature currentsensor, and disabling of the motor may be effected only if both loopsare disabled.

Preferably the motor has two opposed series field windings and a meansfor disabling the motor is constituted by a relay arranged to shortcircuit these windings together, so that substantially zero field fluxresults.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS FIG. 1 is a diagram of atypical pulse controlled motor and its associated driving circuit; and

FIG. 2 is a schematic diagram of a safety circuit for use with thecircuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates, in the main,a well known configuration of a motor and its control circuit. A battery1 can supply current to a series wound motor 2 consisting of an armature3 and two opposed series field windings 4 and 5. One field winding ofthe motor is energised for each direction, forward or reverse, ofmovement of the motor. Two coils 6 and 6a are in series with thearmature to provide magnetic flux corresponding to the armature currentto respective regulating circuits described hereinafter. A relay,usually called line contactor, 7 has its contacts in series with thebattery. One of two similar control circuits 8 and 9 is associated witheach field winding. These chopper circuits are each comprised of achopper circuit 10, a trigger circuit 11 and a current sensing circuit12 of which one of the coils 6 and 6a forms a part.

The chopper circuit 10 consists principally of a thyristor 13, which isrendered conductive, allowing armature current to flow in the motor, bya pulse to its gate, a thyristor 14, which is rendered conductive by apulse to its gate and a capacitor 15 which is connected between theanodes of the thyristor and causes the extinction of the conduction ofone thyristor when the other thyristor, whichever it may be, is renderedconductive. For this purpose it is desirable to charge the capacitortowards the batterys voltage when the thyristor 13 is conductive andaccordingly the circuit includes a transformer, whose primary 16 is inseries with the thyristor l3 and whose secondary 17 is in series with adiode 18 in a branch parallel to the thyristor 14.

In the current sensing circuit the respective coil, 6 or 6a, producesmagnetic flux through a magnetically sensitive resistor arranged in apotential dividing circuit energised by a constant voltage. The triggercircuit is responsive to the voltage across part of the divider anddevelops an operating pulse followed by an extinguishing pulse which arerespectively fed to the thyristors l3 and 14 of the associated choppercircuit when the sensed voltage falls below a value (such as twelvevolts) associated with a relatively small armature current.

The circuit as thus far described by way of example corresponds to therelevant parts of the circuit described in the aforementioned US. Pat.No.35626l6 the disclosure of which is incorporated herein by reference.

The circuit of FIG. 1 also includes a relay 19 of which the coil 20 isnormally energized to keep the relay open. The relay is closable toshort circuit the ends, of the field windings, remote from the armature3. Any current circulating in the thereby constituted loop of twowindings will cause equal and opposite torques on the armature, so thatthe motor is then effectively disabled.

FIG. 2 illustrates, partly schematically, the safety circuit. Itcomprises a square wave transistor oscillator 21 which requires for themaintenance of oscillations an untrammelled passage of signals around atleast one of two feedback loops 22 and 23. One loop 22 is completed by atransistor amplifying circuit 24 which is coupled to a bias circuit.This bias circuit is connected via an OR circuit 26 to the anodes of themain thyristor 13 in each chopper circuit and functions to disable theassociated oscillatory loop by turning off the transistor circuit if thepotential at the anode junction is low. A continuously low potential atthis junction is a condition capable of causing continuous energisationof a low resistance motor circuit and thereby either damage to thatcircuit or run-away of a truck driven by the motor or both. The otherloop is completed by a transistor amplifying circuit 27 which is coupledto a bias circuit 28. This bias circuit is energised from the output ofthe armature current sensing circuit 12 and if the armature currentreaches a limit denoted by an output, from the sensing circuit, of 16volts, the bias circuit turns off the circuit and thereby disables theassociated loop.

Both oscillatory loops, which have been simplified in FIG. 2 for ease ofunderstanding, include the power transistor 29. This transistor drivesthe coils 7a, of the line contactor 7 and the relay 19 in parallel.These coils are jointly in series with the normally closed contacts 30of a normally energised relay 31 which is controlled by the oscillator.The relay remains energised provided that the oscillator 21 remainsoscillatory. This function can simply be achieved by disposing therelays coil 32 in series with the collector/emitter circuit of aswitching transistor in the oscillator and providing a free-wheel diode33 across the coil so that provided the transistor switches onrepeatedly in accord with the operation of the oscillator the coilreceives sufficient current to keep its contacts closed. Variousadequate arrangements for this purpose will readily occur to thoseskilled in the art.

Thus the safety circuit is provided principally to guard against thecondition of continuously conductive thyristors and high armaturecurrent. Should this condition occur, both the possible oscillatoryloops are disabled, the power transistor and the transistor switch off,and the coils are de-energised. No current can reach the motor and anyresidual current in the motor causes minimal torque owing to the shortcircuiting together of the opposed field windings.

It will be appreciated that the disabling of the loops is not dependenton any particular construction of armature current sensing circuit oruse of any particular chopper circuit but merely consists in the sensingof an appropriate operating voltage. Thus the general principle ofdisabling the loop can be extended to almost any part of the motor orits associated circuit.

Normally the pulses energising the motor each last about I millisecond;the square wave oscillator can, for example, generate pulses which lastfor 120 micro seconds, separated by spaces of between ten and fortymicro seconds. Thus these latter pulses are at a frequency high relativeto the pulse-operating frequency of the motor.

I claim:

1. A circuit comprising a motor, a control circuit for energising themotor with pulses of current, a safety circuit for monitoring selectedelements of the control circuit and a relay for disabling the motor, inwhich the safety circuit is oscillatory at a relatively high frequencyand includes at least one closed loop and means for disabling the loopon the occurrence of predetermined conditions in the control circuit;and in which the said relay is operative to disable the motor in theabsence of oscillation by the oscillatory safety circuit.

2. A circuit according to claim 1 in which the control circuit includesa thyristor in series with the motor and a sensing circuit coupled tosense the motors armature current and provide an output dependentthereon, in which the oscillatory safety circuit includes twoalternative oscillatory loops, means for disabling one of the loops whensaid thyristor is conductive and means for disabling the other loop whensaid output reaches a preset level.

3. A circuit according to claim 1 in which the said relay is normallyopen and energised and is arranged when closed to short circuit togetherthe ends of two opposed series field windings of the motor.

4. A circuit comprising a direct current motor with an armature and atleast one field winding; a control circuit for energising the motor withpulses of current, said control circuit including a first thyristor inseries with said field winding; means for rendering said thyristorintermittently conductive; means for sensing current in said armature;switch means having an open energised state and a closed de-energisedstate, said switch means effecting disabling of said motor when in saidclosed de-energised state; a safety circuit comprising an oscillatorhaving first and second alternative feedback loops; bias means couplingsaid thyristor to said first loop for disabling same on conduction ofsaid thyristor; further bias means coupling said means for sensing tosaid second loop for disabling same on occurrence of a predeterminedlevel of armature current; and means coupling said oscillator to saidswitch means for de-energising said switch means in the absence ofoscillation by said oscillator.

5. A circuit according to claim 4 in which said motor has two seriesfield windings which are jointly coupled at one end to the armature andsaid switch means includes a relay for connecting the other ends of thewindings together.

6. A circuit comprising a direct current source, a direct current motorwith an armature and two opposed series field windings having first endsjointly coupled to the armature and second ends, current choppercircuits between each of said second ends and said source of directcurrent, means for selecting one or other of said current choppercircuits for operation, means for supplying pulses to one or other ofsaid windings selectively, and a relay having an open energized stateand a closed de-energized state, said relay in said closed stateconnecting said second ends together.

1. A circuit comprising a motor, a control circuit for energising themotor with pulses of current, a safety circuit for monitoring selectedelements of the control circuit and a relay for disabling the motor, inwhich the safety circuit is oscillatory at a relatively high frequencyand includes at least one closed loop and means for disabling the loopon the occurrence of predetermined conditions in the control circuit;and in which the said relay is operative to disable the motor in theabsence of oscillation by the oscillatory safety circuit.
 2. A circuitaccording to claim 1 in which the control circuit includes a thyristorin series with the motor and a sensing circuit coupled to sense themotor''s armature current and provide an output dependent thereon, inwhich the oscillatory safety circuit includes two alternativeoscillatory loops, means for disabling one of the loops when saidthyristor is conductive and means for disabling the other loop when saidoutput reaches a preset level.
 3. A circuit according to claim 1 inwhich the said relay is normally open and energised and is arranged whenclosed to short circuit together the ends of two opposed series fieldwindings of the motor.
 4. A circuit comprising a direct current motorwith an armature and at least one field winding; a control circuit forenergising the motor with pulses of current, said control circuitincluding a first thyristor in series with said field winding; means forrendering said thyristor intermittently conductive; means for sensingcurrent in said armature; switch means having an open energised stateand a closed de-energised state, said switch means effecting disablingof said motor when in said closed de-energised state; a safety circuitcomprising an oscillator having first and second alternative feedbackloops; bias means coupling said thyristor to said first loop fordisabling same on conduction of said thyristor; further bias meanscoupling said means for sensing to said second loop for disabling sameon occurrence of a predetermined level of armature current; and meanscoupling said oscillator to said switch meAns for de-energising saidswitch means in the absence of oscillation by said oscillator.
 5. Acircuit according to claim 4 in which said motor has two series fieldwindings which are jointly coupled at one end to the armature and saidswitch means includes a relay for connecting the other ends of thewindings together.
 6. A circuit comprising a direct current source, adirect current motor with an armature and two opposed series fieldwindings having first ends jointly coupled to the armature and secondends, current chopper circuits between each of said second ends and saidsource of direct current, means for selecting one or other of saidcurrent chopper circuits for operation, means for supplying pulses toone or other of said windings selectively, and a relay having an openenergized state and a closed de-energized state, said relay in saidclosed state connecting said second ends together.